Image producing machines such as electronic printers and copiers are frequently of the electrophotographic type. In electrophotographic printers, a print is produced by creating an image of the print on a photoreceptive surface, developing the image and then fusing the image to print material. In machines which utilize plain bond paper or other ordinary image receiving material not specially coated, the electrophotographic process is of the transfer type where a photoreceptive material is placed around a rotating drum or arranged as a belt to be driven by a system of rollers. In the typical transfer process, photoreceptive material is passed under a stationary charge generating station to place a relatively uniform electrostatic charge, usually several hundred volts, across the entirety of the photoreceptive surface. Next, the photoreceptor is moved to an imaging station where it receives light rays which are modulated in accordance with the data to be printed. The light generator may produce laser beams, it may be an array of light-emitting diodes, or it may be any other suitable light source. The light rays are directed to the photoreceptor and cause it to bear a charge pattern which is a latent image of the information used to modulate the light rays. Modulation is usually derived from a character generator which is driven by image pattern data frequently produced by a computer and held in digitized form in memory.
After producing an image on the photoreceptor, the next step in the electrophotographic process is to move the image to a developing station where developing material called toner is placed on the image. This material may be in the form of a colored powder which carries a charge and is electrostatically attracted to those areas which it is desired to develop. Thus, pels representing character printing should receive heavy toner deposits, white background areas should receive none, and gray or otherwise shaded portions should receive intermediate amounts. To aid in attaining these results, a bias voltage is usually placed on the developer station to alter the magnitude of electrostatic fields in the development zone. Thus, the bias voltage is established at a level which provides a field development vector to move the charged toner particles away from the developing station toward the areas to be developed while simultaneously establishing an electrostatic field development vector to move the charged toner particles away from the background areas toward the developing station.
The photoreceptor, with a developed image, is moved from the developer to a transfer station where print receiving material, usually paper, is juxtaposed to the developed image. A charge is placed on the backside of the print paper so that when the paper is stripped from the photoreceptor, the toner material is held on the paper and removed from the photoreceptor. Any toner remaining on the photoreceptor after transfer is removed by a cleaning station before the photoreceptor is reused.
The electrophotographic process is frequently used as a copy process as well as a printing process. In the copy process, a document to be copied is placed on a document glass and light is reflected from the original onto the photoconductor. Since white areas of the original document reflect large amounts of light, the photoreceptive material is discharged in white areas to relatively low levels while the dark areas continue to contain high voltage levels even after exposure. At the developing station, the toner material carries a charge opposite in polarity to the charge pattern on the photoreceptor. Because of the attraction of the oppositely charged toner, it adheres to the surface of the photoreceptor in large amounts on the undischarged areas representing the dark areas of the original document. This process is called a charged area development (CAD) process since heavy toner deposits are made on the heavily-charged areas of the photoconductor after exposure.
In electrophotographic printers, a CAD process can be used, but it is often preferable to use a discharged area development (DAD) process, primarily because line and character printing results are usually improved. In the DAD process, the light-generating source, such as a laser beam or an array of light emitting diodes, etc., discharges the photoconductor in those areas which are desired to be developed; thus, the highly-charged areas of the photoconductor represent white background, whereas the discharged areas represent areas in which toner is to be deposited. In the DAD process, toner material carries a charge of the same polarity as the charge pattern on the photoreceptor. Because of the repulsion of the similarly charged toner, it does not adhere to the highly-charged background areas, but instead deposits in the more lowly charged discharged areas.
In many printers, a dual component developing mix is utilized in order to produce the desired charge level on the toner and/or to move the toner to the development zone. For example, in many magnetic brush developers, magnetic beads and toner particles comprise the developer mix. The carrier material and toner particles are churned in the developer to produce a triboelectric charge such that the toner particles are attracted to the carrier. The magnetic carrier material is then moved by magnetic fields to the development zone carrying the charged toner particles therewith. As described above, toner particles are then developed onto the photoconductor and eventually transferred to print paper and moved out of the machine. Therefore, a need to replenish toner particles in order to maintain proper toner particle concentration in the developer mix is essential to good machine operation. Other printers use a monocomponent developing material, toner alone, which receives a charge and develops out onto the photoconductor. Again, toner supply in the developer must be replenished so that the machine can continue to produce output.
While the background of the invention has been provided with reference to electrophotographic printers, the problems of developing a desired toner mass on the print are found in other non-impact electronic printing processes such as ion deposition and magnetic. The invention herein applies to these other processes as well.
One of the best toner concentration control systems found in the prior art is often called the "toner patch" control system. In that system, a small patch of toner is developed on the photoconductor and its reflectivity is sensed and compared to a reference stored in memory. The difference is then used to control the replenishment apparatus to reestablish proper toner concentration.
An important refinement of the patch control system is the use of control ratios as opposed to a difference control. In the ratio system, the reflectivity of bare photoconductor is sensed and compared to the reflectivity of the toned patch. That ratio is compared to a desired control ratio and the difference used to reestablish proper toner concentration. The desired control ratio was not a calculated quantity in prior art systems, it was empirically determined from test data by using an average from several different sensing units or by manual adjustment of the sensing circuit each time a sensing unit was changed. The desire to calculate the control ratio setpoint and avoid the need for manual adjustment stimulated the discoveries which are central to the instant invention. It should be observed that the ratio system of the prior art provided better results than the simple difference control since it included a signal derived from the actual bare photoconductor in use. The improvement was derived from the fact that as the photoconductor surface reflectance changed through usage, the ratio system would automatically compensate for the change.
The invention herein, however, recognizes that previous ratio control algorithms did not consider the effect of toned patch reflectivity when toner reflectivity is high and/or where toner coverage of the patch is high, that is, for high optical density development. In such case, the previous ratio control techniques are not self-compensating for photoconductor degradation. Moreover, previous control algorithms did not take into account the optical difference from sensor to sensor, therefore necessitating the addition of expensive optical components to smooth out such difference, or necessitating manual adjustment of the system when it was manufactured and whenever the sensor unit was changed.
It is the object of the invention to provide a toner patch sensing system that operates in a self-compensating manner for high optical density development
It is another object to provide a toner patch control system that will provide accurate results for toner whose reflectivity is high, as is often the case with non-black toners and is sometimes the case for black toners.
It is another object of the invention to provide a toner patch control system that provides accurate results regardless of the reflectivity of the particular photoconductor in use and regardless of degradation in photoconductor reflectivity.
It is still another object of the invention to provide a toner patch control system that is insensitive to the particular installed sensor unit such that no manual adjustment of the system is needed or, at the most, simple operator controlled parameter changes can insure continued non-deviant operation when sensor units are changed.
It is another object of the invention to provide an accurate toner patch sensing system in order to keep toner mass developed at the proper level by adjusting one or more factors affecting toner mass developed including toner concentration, charge level on the photoreceptor, illumination intensity, or developer bias voltage.
It is still another object of this invention to provide an accurate toner patch sensing system for toners regardless of color and for monocomponent developers as well as multiple component developer mixes.